The present invention relates to a method of making a semiconductor device which has multi-levels of polycrystalline silicon (hereinafter referred to as "polysilicon") conductors with insulation between the conductors. More particularly, the present invention relates to such a method which allows for control of the thickness of the insulation between the outer conductor and both the inner conductor and the substrate.
In semiconductor integrated circuits, the trend has been to the use of conductive polysilicon for conductors, such as the gates of MOS field effect transistors and as interconnections between devices in the circuit. There are some types of integrated circuits, such as floating gate memory devices and charge coupled devices, which use multi-levels of such polysilicon conductors with insulation between the levels. In some of these types of integrated circuits, the second or outer level of the polysilicon overlaps and is insulated from a portion of the substrate as well as overlaps and is insulated from the first or inner level polysilicon. The insulation between the levels of the polysilicon and the substrate is generally thermally grown silicon oxide. It is well known that the thickness of the oxide thermally grown over a given time period on phosphorous doped polysilicon will be different from that grown on single crystalline silicon used as the substrate. Also, the thickness of the oxide grown over the edge of a defined area of polysilicon, such as the edge of a floating gate, will also be different. In addition, the insulating properties of silicon oxide thermally grown from polysilicon are inferior to those of silicon oxide grown from single crystalline silicon. In particular, oxides grown from polysilicon have a lower dielectric strength and exhibit enhanced Fowler-Nordheim tunneling current for a given applied electrical field. These properties are further degraded by the presence of crossover points where the outer polysilicon layer overlaps an edge or sidewall of the inner polysilicon layer. Therefore, it is desirable to have a method of making a semiconductor device having multi-levels of polysilicon conductors which permit tailoring the thicknesses of the oxides between the outer level and each of the inner level and the substrate to achieve desired electrical characteristics.